An objective of radiation therapy is to maximize the amount of radiation to a target volume (e.g., a cancerous tumor) and minimize the amount of radiation to healthy tissues and critical structures. The process of identifying the precise location of the target volume immediately prior to a dose of therapeutic radiation is key to the objective. Since each patient is treated over 30 to 40 fractionated sessions, then the time allowed for each session is relatively short, e.g. 10 to 15 minutes, so the process must be fast as well as accurate.
In the case of electronic portal imaging, megavolt therapeutic X-rays emerging from the patient can be used to generate images. However, this method of target location generates images of low contrast and quality, in addition to incidentally damaging healthy tissue. As a result, imaging with megavoltage (MV) radiation is used primarily for portal verification, that is, to confirm that the treatment volume is being radiated.
Radiotherapy simulator machines have been used to perform the pretreatment analysis of the target volume before a radiotherapy treatment machine applies the therapeutic radiation. However, traditional radiotherapy simulator machines use bulky image intensifier tube detectors to capture images of the treatment volume. These image intensifier tube detectors have the disadvantage of being very large relative to their imaging area. They also have image spatial distortions from their spherical shaped input surface and the orientation of the intensifier tube with the Earth's magnetic field.